1. Field of the Invention
The present invention relates to electrochemical reduction of metal oxides.
The present invention relates particularly to continuous and semi-continuous electrochemical reduction of metal oxides in the form of powders and/or pellets to produce metal having a low oxygen concentration, typically no more than 0.2% by weight.
The present invention was made during the course of an on-going research project on electrochemical reduction of metal oxides being carried out by the applicant. The research project has focussed on the reduction of titania (TiO2).
2. Description of Related Art
During the course of the research project the applicant has carried out experimental work on the reduction of titania using electrochemical cells that include a pool of molten CaCl2-based electrolyte, an anode formed from graphite, and a range of cathodes.
The CaCl2-based electrolyte was a commercially available source of CaCl2, namely calcium chloride dihydrate, which decomposed on heating and produced a very small amount of CaO.
The applicant operated the electrochemical cells at potentials above the decomposition potential of CaO and below the decomposition potential of CaCl2.
The applicant found that at these potentials the cells could electrochemically reduce titania to titanium with low concentrations of oxygen, ie concentrations less than 0.2 wt %.
The applicant does not have a clear understanding of the electrochemical cell mechanism at this stage.
Nevertheless, whilst not wishing to be bound by the comments in the following paragraphs, the applicant offers the following comments by way of an outline of a possible cell mechanism.
The experimental work carried out by the applicant produced evidence of Ca metal dissolved in the electrolyte. The applicant believes that the Ca metal was the result of electro-deposition of Ca++ cations as Ca metal on the cathodes.
As is indicated above, the experimental work was carried out using a CaCl2-based electrolyte at a cell potential below the decomposition potential of CaCl2. The applicant believes that the initial deposition of Ca metal on a cell cathode cell was due to the presence of Ca++ cations and O−− anions derived from CaO in the electrolyte. The decomposition potential of CaO is less than the decomposition potential of CaCl2.
In this cell mechanism the cell operation is dependent on decomposition of CaO, with Ca++ cations migrating to the cell cathode and depositing as Ca metal and O−− anions migrating to the anodes and forming CO and/or CO2 (in a situation in which the anode is a graphite anode) and releasing electrons that facilitate electrochemical deposition of Ca metal on the cathode.
The applicant believes that the Ca metal that deposited on the cathode participated in chemical reduction of titania resulting in the release of O−− anions from the titania.
The applicant also believes that the O−− anions, once extracted from the titania, migrated to the anode and reacted with anode carbon and produced CO and/or CO2 and released electrons that facilitated electrochemical deposition of Ca metal on the cathode.
The applicant operated the electrochemical cells on a batch basis with titania in the form of pellets and larger solid blocks in the early part of the work and titania powders in the later part of the work.
The applicant also operated the electrochemical cells on a batch basis with other metal oxides.
Whilst the research work established that it is possible to electrochemically reduce titania (and other metal oxides) to metals having low concentrations of oxygen in such electrochemical cells, the applicant has realised that there are significant practical difficulties operating such electrochemical cells commercially on a batch basis.
Nevertheless, in the course of considering the results of the research work and possible commercialisation of the technology, the applicant realised that commercial production could be achieved by operating the electrochemical cell on a continuous or semi-continuous basis with metal oxide powders and/or pellets being transported through the cell in a controlled manner and being discharged in a reduced form from the cell.